Dolomite delivers The existence of specialised minerals in ancient reefs suggests the Earth's oceans contained very little oxygen 650 million years ago, say Australian researchers.

"This is really interesting, because this is around the time multicellular animals started evolving," says University of Melbourne PhD student Ashleigh Hood whose research is published in Geology.

The discovery is based on an analysis of minerals called dolomites that would have grown in seawater.

The dolomites come from recently-discovered fossil reef complexes in the Adelaide fold belt, which forms the Flinders Ranges in South Australia.

"This is a bunch of rocks that are about 800 to 500 million years old. The belt was formed when the Earth's plates pushed together," explains Hood.

The reef, which was once upright in the sea and extended to a depth of one kilometre, is now turned on its side, making it easily accessible to researchers.

"You can walk across it and pick up samples from the shallow end through to the deep end, but it's a kilometre of walking every time you want a rock from the other side," says Hood.

Absence of oxygen

The researchers cut the rocks from the reef complexes into thin slices and put them under a special microscope.

"You shoot a beam of electrons at them, like a TV, and the colour of the resulting image reflects the chemical composition," explains Hood.

It also reveals the shape and direction of mineral growth.

"The shape of the mineral is like a sword, it grows upwards but a centimetre of dolomite takes tens of thousands, to hundreds of thousands of years to grow."

Dolomite is a cement-like mixture composed of calcium-magnesium carbonate. It does not precipitate from modern seas, but Hood and colleagues found it in their rocks.

These rocks, which would have been in shallow seas, were also found to contain high levels of iron.

"Because the dolomite mineral has so much iron in it, it suggests that iron was present in solution in the ocean," she says.

"The only time you get iron dissolved in the ocean is when there's no oxygen, because otherwise it would form deposits - it would drop out as iron ore."

Cold climate

The Adelaide fold belt reef is believed to have grown in the interglacial period between two massive ice ages.

According to Hood, it is very likely that the ocean circulation shut down, or slowed down during these cold times, which could have led to the ocean becoming stagnant and stratified (where different depths have different conditions).

This would have led to low oxygen levels and anoxic conditions.

Professor Maurice Tucker of the University of Durham says the Australian research is very exciting and that the discovery of a geological situation where the sediments are both limestone and dolomite is significant.

"There is clear evidence for the limestone being precipitated on the seafloor (as aragonite), with the dolomite being deposited very soon after. "

"Aragonite precipitates first - probably in very shallow oxygenated seawater, as now - but then dolomites precipitates when the water goes anoxic.

"This may be below a certain depth if the sea is stratified, or it may be within cavities in the depositing limestone," he says.

Tucker adds that microbes would also have been around to create the anoxic conditions that are implied by the presence of iron in the dolomite.

Oxugen and the evolution of life

Hood has recently returned from Namibia where she collected similar rocks of the same age, but she has "yet to cut them up".

She is also investigating similar younger reef formations in an attempt to attach a timeline to changes in seawater chemistry.

"There's a big debate around when the iron started to drop out [of solution] ... No one really knows if oxygenation was a gradual or sudden thing."

"[Our work] does have a bearing on evolution of animals, because it is thought animals can't evolve unless there is a certain amount of oxygen."

"I want to look at all of the oceans for this time period and make a timeline to see when dolomite forms and when it doesn't, so I can graph how much oxygen there is in the ocean."

"If you compare that with animals, you can see if there is a correlation with adapting to oxygen," says Hood.

In evolutionary terms, the timing is significant because it directly precedes the 'Cambrian explosion' when most animal groups appeared for the first time.

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